With the development of digitalization in the information-oriented society, there has been a demand for a writable medium which offers higher density recording and readout.
Under such a circumstance, there have been studies for various medium structures for a writable optical recording medium so as to improve its storage capacity. Also, in order to realize high density recording and readout, various methods have been attempted; for example, (i) a method of shortening wavelength of a laser beam used for recording and readout, (ii) a method of increasing NA (Numerical Aperture) of the objective lens used for condensing light onto the optical information recording medium, (iii) a method of providing multi-layered recording section to an optical information recording medium, and iv) a method of reducing the practical diameter of beam spot of a laser beam by providing a mask layer (also called a light-shutter layer), which blocks a part of the original diameter of the light.
This method of reducing the practical diameter of beam spot of a laser beam by providing a mask layer is described in Japanese Laid-Open Patent Application No. 5-12673/1993 (Tokukaihei 5-12673; published on Jan. 22, 1993) and Japanese Laid-Open Patent Application No. 5-12715/1993 (Tokukaihei 5-12715; published on Jan. 22, 1993), for example.
More specifically, Japanese Laid-Open Patent Application No. 5-12673/1993 discloses an optical disk device for performing recording and readout by using change of reflectivity which is occurred in a concavo-convex state due to pits on the substrate of the optical disk. This optical disk device is provided with a temperature dependent light shutter layer on the medium layer (on a surface of the disk to which a laser beam is emitted from the objective lens) for signal recording. The temperature dependent light shutter layer, which absorbs readout laser wavelength at ordinary temperature, stops absorption when the temperature rises by readout laser power, and starts the absorption again as the temperature drops after the readout light condensing spot.
Japanese Laid-Open Patent Application No. 5-12673/1993 further discloses another arrangement of an optical disk device for performing recording and readout by using change of reflectivity caused by phase change of the recording material. This optical disk device also includes the temperature dependent light shutter layer on the medium layer (on a surface of the disk toward which a laser beam is emitted from the objective lens) on which signals are recorded, which absorbs readout laser beam at ordinary temperature, and stops absorption when the temperature rises from readout laser power, and starts the absorption again as the temperature drops after passing through the readout light condensing spot.
Further, Japanese Laid-Open Patent Application 5-12715/1993 discloses an optical recording medium including a recording film on the substrate, in which a light shutter layer is provided on the recording film so that light irradiation for reading or recording incident on the recording film is blocked just before entering to the recording film. The light shutter layer contains a thermochromic material as its main component, which gives a color to the light shutter layer for preventing transmission of light before the light irradiation for reading is carried out, and then, when the light irradiation for reading is carried out and temperature rises, the central portion of the irradiation area in the light shutter layer decolorizes and becomes light-transmissive.
In the foregoing methods, the optical information recording medium having concave-convex pits or a recording film on the substrate is provided with a mask layer on surface receiving incident light toward the pits or the recording film. The mask layer is generally formed by a thermochromic material or a phase change material. In the central portion of the irradiation part of the mask layer, the temperature rises due to incident of the light irradiation for reading etc., and therefore the central portion of the irradiation part is optically or thermally reacted and partly loses the color to be light-transmissive. On the other hand, the temperature rises only slightly or does not rise at all in a portion receiving weak incident light, such as periphery of the irradiation part or a portion where light is not incident, and therefore, such a portion has a light blocking property. As a result, the practical spot size can be reduced to be equal to the size of the central part of the irradiation part. More specifically, the mask layer allows light transmission only in a portion having high light intensity distribution, thus practically reducing the spot diameter of the incident light. Accordingly, it becomes possible to carry out recording and readout with a small pit, which enables recording and readout of an optical information recording medium with high density. Note that, the central part of a rotating optical disk is an upper section of a readout beam spot 33 in FIG. 13 (in FIG. 13, high temperature area 33a).
The thermochromic material or the phase change material for the mask layer (light shutter layer) disclosed in Japanese Laid-Open Patent Application Nos. 5-12673/1993 and 5-12715/1993 shows the effect as a mask when the material is dissolved at a certain temperature. Since the material in the dissolved state has high fluidity, the original composition and formation of the material can be easily changed. Thus, in an optical information recording medium having such a mask layer showing the effect as a mask under a circumstance of a certain temperature, the mask effect will be gradually reduced due to the change of the original composition and formation, after a large number of recordings and/or readouts is performed; and eventually, the effect will be completely lost. Thus, the foregoing conventional optical information recording medium has insufficient durability.
Further, Japanese Laid-Open Patent Application No. 5-12673/1993 has such description that “This temperature dependent light transmittance changeable medium is formed by, for example, a polymeric material or an organic material . . . , a material whose transmittance is increased in a high temperature area may be adopted for such a material, for example. Such a change in transmittance may be caused by using a material whose light transmittance is increased from dissolution of the material, or may be caused by changing regularity of molecular alignment of a liquid crystal material. Further, the material may be a phase change material or the like, and the light transmittance of such a material can be changed, for example, by heating and cooling a chalcogenide in an amorphous state to cause crystallization.” However, this description fails to describe a specific example of the material whose light transmittance is increased in a high temperature area. For this reason, the invention disclosed in Japanese Laid-Open Patent Application No. 5-12673/1993 cannot be realized by those in the art, based on common technologies at the time when Japanese Laid-Open Patent Application No. 5-12673/1993 is written or published.